This invention pertains to the art of rotational molding or casting and more particularly to a method and apparatus for the rotational molding or casting of thin walled items.
The invention is particularly applicable to a rotational molding machine for the manufacture of thin walled or hollow items shaped in conformance with a mold. However, it will be appreciated to those skilled in the art that the invention could be readily adapted for use in other environments as, for example, where similar methods or apparatus are employed to manufacture items.
Numerous types of apparatus and methods for molding thin walled articles are known in the art. A common rotational molding machine is one which fixedly mounts a plurality of mold carrying arms on a turret which rotates to move the arms from one operating station to another. At the operating stations the molds on the arms are loaded with a thermoplastic or like material, rotated and heated in an oven chamber, and cooled at a cooling station. During rotation and heating, the mold is typically rotated about two axes normal to each other.
The deficiencies found in these types of rotational molding machines are such that the machines themselves are of limited economic and practical value due to a number of inherent problems. A principal problem is cost and size. The complexity of a machine which must carry a plurality of arms on a turret through different operating stations has produced an apparatus which is excessively costly to manufacture and maintain. In addition, the physical size of the apparatus requires a great amount of factory space.
An additional problem in prior rotational molding machines has been the extremely short life span of bearings which support the mold carrying arms. In prior art machines when a mold is being rotated about two normal axes and heated, the bearings have been in the oven chamber or heating station. Likewise, when the mold is cooled. Such temperature changes have had an extremely debilitating effect on the bearings.
A further problem in prior rotational molding machines has been the difficulties with mold loading, mounting and removal. A mold must be loaded with plastic material and then mounted on a turret arm. Where a plurality of turret arms are involved, loading, mounting and removal of molds has been particularly time consuming and inefficient.
The present invention contemplates a new and improved apparatus and method which overcomes all of the above referred to problems and others to provide a new apparatus and method for rotational molding which is simple in design, economical to manufacture, readily adaptable to a plurality of uses with molds having a variety of dimensional characteristics, easy to install, easy to maintain and which provides improved molded articles.